Field of the Invention
The present invention relates to a water heater, and particularly to a water heater of an exhaust suction type which suctions and exhausts combustion gas to the outside of the water heater.
Description of the Background Art
It has been known that oscillating combustion occurs depending on a combustion condition in a burner having a large number of burner ports. Namely, when flame is disturbed by disturbance of a flow velocity (an amount per unit time of injection from burner ports) of an air fuel mixture, an amount per unit time of heat generation (a rate of heat generation) by flame fluctuates. When a cycle of fluctuation in pressure caused by this fluctuation coincides with a natural frequency of a burner, this frequency is excited and oscillating combustion occurs. When oscillating combustion occurs, a combustion state becomes unstable and noise may be generated due to oscillation of the burner.
A burner capable of achieving low-NOx combustion and high-load combustion has recently been demanded with reduction in size of the burner. It has been known, however, that oscillating combustion is likely particularly in such a burner (see, for example, Japanese Patent Laid-Open No. 2000-249306).
It has conventionally empirically been known that oscillating combustion is likely to occur when burner ports are equal in size (cross-sectional area). Therefore, for example, measures for suppression of oscillating combustion based on an effect of interference between flames by making sizes of burner ports of combustion tubes non-uniform have been taken. A mechanism of occurrence of oscillating combustion, however, has not sufficiently been clarified. Actually, measures for suppression of oscillating combustion are taken for each individual burner through trial and error by experientially changing a combustion condition, a shape of a burner and the like.
As disclosed in Japanese Utility Model Publication No. 63-24347 and Japanese Utility Model Publication No. 8-585, on the other hand, a water heater of an exhaust suction type having air blow means on a downstream side of an exhaust flow path of a burner in order to suction and exhaust combustion gas to the outside of the water heater has been known.
Conventionally, there has been insufficient discussion of measures for suppression of oscillating combustion when a burner is reduced in size and when low-NOx combustion and high-load combustion are desired in a water heater of an exhaust suction type.
Measures for suppression of oscillating combustion have been discussed for a water heater of a forced exhaust type in which combustion gas is exhausted to the outside as a fan forces outside air into the inside. The present inventors have found, however, that measures similar to those for the water heater of the forced exhaust type alone cannot necessarily sufficiently suppress oscillating combustion in a water heater of an exhaust suction type particularly when a burner is reduced in size and low-NOx combustion and high-load combustion are desired.